1. Field of the Invention
The present invention concerns a method of manufacturing a 2-layered flexible substrate and, more specifically, it relates to a method of manufacturing a 2-layered flexible substrate capable of easily forming a copper conductor layer of more intact and adhesive property upon forming a copper conductor layer on an insulation film by adopting a dry plating method, an electroless plating method and an electric copper plating method.
2. Statement of the Prior Art
Substrates used for manufacturing flexible wiring boards are generally classified into a 3-layered flexible substrate in which a copper foil as a conductor layer is appended on an insulation film by using an adhesive and a 2-layered flexible substrate in which a copper conductor layer is formed directly on an insulation film without using an adhesive by a dry plating method or a wet plating method.
In a case of using the 3-layered flexible substrate, a 3-layered flexible wiring board can be manufactured by forming a desired wiring pattern on the substrate by a subtractive method. In the case of using the 2-layered flexible substrate, a 2-layered flexible wiring board can be manufactured by forming a desired wiring pattern on the substrate by a subtractive method or an additive method. Generally, use of the 3-layered flexible substrate which employs simple manufacturing method and can be manufactured at a reduced cost has been predominant.
By the way, along with the increasing density of electronic equipments in recent years, narrower pitch has been demanded for the width of wirings in wiring boards. However, upon manufacturing wiring boards, when wiring portions are formed by etching a copper conductor layer formed on an insulation film of a substrate in accordance with a desired wiring pattern, since a so-called side etching is caused in which the lateral side of the wiring portion is etched, the wiring portion tends to have a diverging trapezoidal cross sectional shape. Accordingly, since the width of the wiring pitch is excessively large, if etching is conducted till electric insulation is ensured between the wiring portions, there is a limit for narrowing the pitch for the width of the wiring portions in the wiring board in the use of a generally employed 3-layered flexible substrate in which a copper foil of 35 .mu.m thickness used generally is appended.
In view of the above, it has been attempted to use a substrate in which a thin copper foil of 18 .mu.m or less is appended instead of a conventional substrate in which a copper foil of 35 .mu.m thickness is appended, thereby decreasing the diverging width caused by the side etching to narrow the pitch in the wiring portion of a wiring board. However, since the copper foil of such a reduced thickness has less rigidity and poor handlability, it has been adopted a method of once appending a reinforcing material such as an aluminum carrier to the copper foil to increase the rigidity, then appending the copper foil and the insulation film and then removing the aluminum carrier. However, this method involves a problem of taking much labor and time and is poor in the operation efficiency.
Further, such a thin copper foil also involves a problem in view of manufacturing techniques that defects of film are increased due to scattering of the film thickness, pin holes or occurrence of crackings. Further, as the thickness of the copper foil is decreased, manufacturing thereof becomes more difficult, which increases the manufacturing cost to lose the economical merit of the 3-layered flexible wiring boards because of increase in the manufacturing cost. Particularly, since it has been an increasing demand recently for a wiring board having a wiring portions with such a narrow width and a narrow pitch that can not be manufactured unless a copper foil having a thickness of less than ten and several micrometers, for example, about of several micrometers, the wiring board using the 3-layered flexible substrate involves problems in view of the manufacturing cost as well as the technical problems described above.
Then, it has been noted a 2-layered flexible wiring board using a 2-layered flexible substrate capable of forming a copper deposition layer on an insulation film directly without applying an adhesive. The 2-layered flexible substrate has a copper conductor layer formed directly on the insulation film without an adhesive and, accordingly has a merit capable of reducing the thickness of the substrate itself and can optionally control the thickness of the copper conductor to be deposited. Then, when such a 2-layered flexible substrate is manufactured, electric copper plating is usually adopted as a means for forming an inexpensive copper conductor layer of a uniform thickness on the insulation film and, generally, a thin underlying metal layer is formed on the insulation film to be applied with an electric copper plated layer to provide the entire surface with electroconductivity, on which electric copper plating is applied.
By the way, the thin underlying metal layer is usually formed on the insulation film by using a dry plating method such as vacuum deposition or ion plating but since a lot of pin holes in the size of from several tens micrometers--several hundreds micrometers are usually formed on the deposited layer obtained by the dry plating method, exposed portions of the insulation film caused by pin holes are often formed on the underlying metal layer.
Heretofore, in the flexible wiring board of this type, it has been considered generally that the thickness of the deposited conductive copper layer required for wirings is appropriately from 35 .mu.m to 50 .mu.m, but since the width of the wirings to be formed is also about several hundred micrometers, defects of the wiring portion induced by the presence of the pinholes of several tens micrometers are scarcely caused.
However, in a case of obtaining a flexible wiring board having a wiring portion of such a narrow pitch as intended in the present invention, it is preferred that the thickness of the deposited layer copper for forming the wiring portion is desirably as thin as possible, namely, less than 35 .mu.m, preferably, less than 18 .mu.m and, most preferably, about 5 .mu.m as described above, so that possibility of causing defects in the wiring portion is increased.
Referring to the above-mentioned situation for a case of manufacturing a 2-layered flexible wiring board, for example, by a subtractive method, using a 2-layered flexible substrate in which a copper conductor layer of a desired thickness is formed on an insulation film formed with an underlying metal layer, the wiring portion is patterned is formed by the following steps.
(1) A resist layer having such a desired wiring pattern that only the wiring portion is masked and a copper conductor layer is exposed at a not-wiring portion is disposed on the copper conductor layer. PA1 (2) The exposed copper conductor layer is removed by chemical etching. PA1 (3) Finally, the resist layer is removed. PA1 forming an underlying metal layer on an insulation film with a deposition layer formed by a dry plating method using at least one material selected from the group consisting of nickel, copper-nickel alloy, chromium and chromium oxide and a copper deposition layer formed by a dry plating method on the above-mentioned deposition layer, then forming a primary electric copper plated deposition layer on the underlying metal layer, then applying a treatment using at least one alkaline solution selected from the group consisting of inorganic alkaline solutions and organic alkaline solutions, then forming an electroless copper plated deposition layer as an intermediate metal layer on the primary electric copper plated deposition layer and, finally, forming a secondary electric copper plated deposition layer on the intermediate metal layer, thereby finally forming a copper conductor layer of 1 to 35 .mu.m thickness on the insulation film.
Accordingly, in a case of using a substrate in which the thickness of the copper conductor layer is formed, for example, as thin as 5 .mu.m, for and manufacturing a wiring board having a narrow wiring width and a narrow wiring pitch, for example, a wiring width of 40 .mu.m and a wiring pitch of 80 .mu.m, the size of large pinholes among pinholes formed in the underlying metal layer of the substrate by the dry plating, reaches the order as large as several tens micrometers to several hundreds micrometers, so that the exposed portion of the insulation film induced by the pinholes cannot be buried scarcely when the electric copper plated deposition layer of about 5 .mu.m thickness is formed and, accordingly, the exposed portion, namely, the defective portion of the conductor layer interferes the wiring portion and the wiring portion is depleted at the position of the pinhole to cause wiring defects or, if not, lead to adhesive failure of the wiring portion.
As a method of solving the foregoing problem, a method has been proposed of forming an underlying metal layer on an insulation film by a dry plating method, and then further applying a deposited copper layer as an intermediate metal layer by electroless plating, thereby covering the exposed portion of the insulation film induced by pinholes. However, although the exposed portion of the insulation film induced by the pinholes can be eliminated to some extent by this method, a plating solution used for the electroless copper plating or a pretreatment solution theref or penetrates through already formed pinholes of various sizes to a position between the insulation film and the underlying metal film, which brings about hindrance for the adhesion of the underlying metal layer and, thus, the adhesion of the conductor layer by electric plating to be formed subsequently, and this method cannot provide a satisfactory countermeasure.